Carbon Paper Gas Diffusion Layer Research Articles

Application of continuum percolation theory for modeling single- and two-phase characteristics of anisotropic carbon paper gas diffusion layers

Percolation theory is used to model intrinsic and relative permeabilities as well as tortuosity in anisotropic carbon paper gas diffusion layers (GDL) and compared with existing results from lattice-Boltzmann (LB) simulations and experimental measurements. Although single- and two-phase characteristics of the carbon paper GDL are mainly affected by medium geometrical and topological properties, e.g., pore-size distribution, connectivity, and pore geometry, analyzing capillary pressure curves implies that the pore-size distribution of the carbon paper GDL is very narrow. This suggests that its effect on tortuosity and wetting- and nonwetting-phase relative permeabilities is trivial. However, integrated effects of pore geometry, surface area, connectivity, and tortuosity on intrinsic permeability might be substantial. Universal power laws from percolation theory predict the tortuosity-porosity and relative permeability–saturation curves accurately, indicating both characteristics not affected by the pore-size distribution. The permeability–porosity relationship, however, conforms to nonuniversality.

Three-dimensional catalyst electrodes based on PtPd nanodendrites for oxygen reduction reaction in PEFC applications

PtPd bimetallic nanodendrites (NDs), with enhanced activities from PtPd over element Pt and unique anisotropic morphology, show potential as catalysts in fuel cell applications. However, the research has been limited to pure materials, and constructing a practical fuel cell catalyst electrode from PtPd NDs still remains as a challenge. In this paper, we demonstrated, for the first time, catalyst electrodes from PtPd NDs for polymer electrolyte fuel cell (PEFC) applications. PtPd NDs are in-situ grown on large-area carbon paper gas diffusion layers (GDLs) and directly employed as cathodes in H2/air PEFCs. The thin catalyst layer with PtPd nanodendrites significantly reduces mass transfer resistance and a higher power performance is achieved than those based on pure Pt nanowires and Pt/C nanoparticle electrocatalysts. The crystal growth mechanisms of this advanced nanostructure on large-area support are also detailed based on the time-dependent experiments and Pd content.

Effects of heterogeneous surface of gas diffusion layers on droplet transport in microchannels of PEM fuel cells

Effects of heterogeneous surface of commercial gas diffusion layers (GDLs) which is made from carbon papers on a droplet transport in microchannels of proton exchange membrane fuel cells are investigated using the volume of fluid method. A random function is used for generating the heterogeneous surface of GDL in terms of roughness density and roughness height, which is derived from experimental data of commercial carbon paper GDLs. Surface tension, and wall adhesion forces are applied by using the volume of fluid method. The hydrophilic walls and hydrophobic GDL are considered. Results show that the heterogeneous surface of GDLs greatly affects droplet transport in the mircochannels. The movement mechanism of large droplet is translational while rolling mechanism is dominant for small droplet movement. Pressure drop and pressure fluctuation of large droplet are reduced after the formation of film flow. Generally, time-averaged pressure drop, droplet elongation, and the amount of remaining water on the GDL increase by increasing roughness density and roughness height, which is undesirable for better water management. It is also observed that the amount of adhered droplet to the hydrophilic surface of microchannel increases by increasing the roughness height and airflow velocity, which is favorable for better water removal. Average velocity of the droplet decreases by increasing roughness parameters for low airflow velocities while it increases for high airflow velocities. The detachment velocity of droplets on the rough GDL is compared with analytical solution for smooth GDLs.

Electrochemical investigation of stainless steel corrosion in a proton exchange membrane electrolyzer cell

The lack of a fundamental understanding of the corrosion mechanisms in the electrochemical environments of proton exchange membrane (PEM) electrolyzer and/or fuel cells (ECs/FCs) has seriously hindered the improvement of performance and efficiency of PEM ECs/FCs. In this study, a stainless steel mesh was purposely used as an anode gas diffusion layer that was intentionally operated with high positive potentials under harsh oxidative environments in a PEMEC to study the corrosion mechanism of metal migration. A significant amount of iron and nickel cations were determined to transport through the anode catalyst layer, the PEM and the cathode catalyst layer during the PEMEC operation. The formation/deposition of iron oxide and nickel oxide on the carbon paper gas diffusion layer at the cathode side is first revealed by both scanning electron microscope and X-ray diffraction. The results indicate the corrosion elements of iron and nickel are transported from anode to cathode through the catalyst-coated membrane, and deposited on carbon fibers as oxides. This phenomenon could also open a new corrosion-based processing approach to potentially fabricate multifunctional oxide structures on carbon fiber devices. This study has demonstrated a new accelerated test method for investigating the corrosion and durability of metallic materials as well.

Effective diffusivity in partially-saturated carbon-fiber gas diffusion layers: Effect of local saturation and application to macroscopic continuum models

Macroscopic continuum models are an essential tool to understand the complex transport phenomena that take place in gas diffusion layers (GDLs) used in polymer electrolyte fuel cells (PEFCs). Previous work has shown that macroscopic models require effective properties obtained under uniform saturation conditions to get a consistent physical formulation. This issue, mostly unappreciated in the open literature, is addressed in detail in this work. To this end, lattice Boltzmann simulations were performed on tomographic images of dry and water-invaded carbon-paper GDL subsamples with nearly uniform porosity and saturation distributions. The computed effective diffusivity shows an anisotropic dependence on local porosity similar to that reported for morphologically analogous GDLs. In contrast, the dependence on local saturation is rather isotropic, following a nearly quadratic power law. The capability of the local correlations to recover the layer-scale properties obtained from inhomogeneous GDLs is checked by global averaging. Good agreement is found between the upscaled results and the diffusivity data of the GDL from which the present subsamples were taken, as well as other global data presented in the literature. A higher blockage effect of local saturation is, however, expected for the under-the-rib region in operating PEFCs.

Effect of through-Plane Polytetrafluoroethylene Distribution in a Gas Diffusion Layer

During operation of a proton exchange membrane fuel cell (PEMFC), the oxygen reduction reaction rate at the cathode is the limiting kinetic step and determines the maximum operating current density. This rate strongly depends on the transport rate of oxygen from the flow channel to the catalyst sites. Although the electrolyte membrane needs liquid water to retain proton conductivity, the oxygen gas transport can be greatly hindered when liquid water accumulates either in the catalyst layer (CL) or the gas diffusion layer (GDL) of the cathode. Consequently, for optimal water management and performance in PEMFCs, water retention/expulsion properties must be carefully balanced.We previously investigated the relationship between the treatment process of polytetrafluoroethylene (PTFE) and the resulting PTFE distribution in through-plane direction of carbon papers [1]. The dependence of PTFE distribution on the PTFE drying conditions was examined using SEM-EDS analysis. The EDS image maps revealed that the PTFE distribution strongly depended on the drying condition; PTFE-drying under atmospheric pressure created highly heterogeneous PTFE distributions in the through-plane direction, with high concentrations near both the upper and lower surfaces, whereas PTFE-drying under vacuum pressure created a relatively uniform PTFE distribution.In this study, in order to investigate the effect of PTFE though-plane distribution on the water discharge characteristic, limiting current measurements were performed under various humidity conditions using a small-scale PEMFC, which has the electrode area of 1 cm2. Based on the limiting current measurement, we could evaluate the gas transport resistance through the carbon paper GDL [2,3]. Because the small-scale cell was applied, we could minimize the effect of the distribution of current density and humidification, that is, the effect of local flooding across the flow channel depending on the change of humidification.ReferencesH. Ito et al., J. Power Sources , 248, 822 (2014).D. R. Baker et al., J. Electrochem. Soc., 156, B991 (2009).N. Nonoyama et al., J. Electrochem. Soc., 158, B416 (2011).

Investigating Inlet Condition Effects on PEMFC GDL Liquid Water Transport through Pore Network Modeling

Polymer electrolyte membrane fuel cells (PEMFCs) are promising electrochemical energy conversion devices due to their high efficiency, zero-local emissions, and rapid start-up capability. Water management is a vital issue in the PEMFC, as the accumulation of water directly affects its operating efficiency. During operation the membrane must be sufficiently humidified in order to achieve high proton conductivity; however, at high current densities, accumulation of liquid water in the cathode gas diffusion layer (GDL) blocks oxygen pathways, preventing this reactant from reaching the catalyst, which in turn hinders performance. Effective water management strategies are essential to improve PEMFC performance and require a detailed understanding of liquid water transport in the GDL.In a fuel cell stack, the cell components are assembled under compressive loads to prevent gas leakage; however, high compression of the GDL may affect cell performance. The non-uniform compression of the GDL, due to its contact with the bipolar plate, significantly alters the microstructure and consequently the dynamics of liquid water transport through the GDL [1]. A number of attempts have been made in experimental visualization of liquid water transport within compressed GDLs, but more work is required to evaluate these effects quantitatively [1-3].The purpose of this study is to numerically investigate the effects of compression on liquid water transport through PEMFC GDLs. In this work, two types of carbon paper GDLs are compressed. At each compression value, volumetric X-ray tomography images of the samples are obtained. The resulting greyscale images are segmented using a novel thresholding algorithm. A watershed algorithm [4] is used to extract pore networks of the GDL. Pore network modeling with invasion percolation is employed to determine the liquid water profile within the GDL over the range of compression states. The results of this study provide a deeper knowledge of how compression and the GDL microstructure affect the movement of liquid water.

Improvement of oxygen diffusion characteristic in gas diffusion layer with planar-distributed wettability for polymer electrolyte fuel cell

Mass transfer characteristics of gas diffusion layer (GDL) are closely related to performance of polymer electrolyte fuel cells. Therefore, it is necessary to clarify the characteristics of water distribution relating to the microscopic conformation and oxygen diffusivity of GDL. A hybrid type carbon paper GDL with planar-distributed wettability is investigated for control of liquid water movement and distribution due to hydrophobic to hydrophilic areas that provide wettability differences in GDL and to achieve enhancement of both oxygen diffusion and moisture retention. Hybrid GDLs with different PTFE content were fabricated in an attempt to improve the oxygen diffusion characteristics. The effects of different PTFE contents on the oxygen diffusivity and water distribution were simultaneously measured and observed using galvanic cell oxygen absorber and X-ray radiography. The PTFE distribution was observed using scanning electron microscopy. The formation of oxygen diffusion paths was confirmed by X-ray radiography, where voids in the hybrid GDL were first formed in the hydrophobic regions and then spread to the untreated wetting region. Thus, the formation of oxygen diffusion paths enhanced the oxygen diffusion. In addition, the effects of local PTFE content in the hydrophobic region and the optimal amount of PTFE for hybrid GDL were elucidated.

Microstructure reconstruction and characterization of PEMFC electrodes

We reconstruct a proton exchange membrane fuel cell (PEMFC) catalyst layer (CL) and a non-woven carbon paper gas diffusion layer (GDL) by specially-designed stochastic methods. The reconstructed microstructures evidently distinguish all the participating components in the composite GDL/CL. Characterization analyses with respect to the reconstructed GDL/CL give important structural properties such as geometrical connectivity of an individual phase, pore size distribution, and volume-specific interfacial area. Self-developed Lattice Boltzmann (LB) models are established to calculate effective transport physical properties including effective thermal/electric conductivity and effective species diffusivity of the reconstructed GDL/CL, and permeability of the reconstructed GDL. Accordingly, we obtain tortuosity values for pore or solid phase in the reconstructed GDL/CL.

Bulk and contact resistances of gas diffusion layers in proton exchange membrane fuel cells

A multi-electrode probe is employed to distinguish the bulk and contact resistances of the catalyst layer (CL) and the gas diffusion layer (GDL) with the bipolar plate (BPP). Resistances are compared for Vulcan carbon catalyst layers (CL), carbon paper and carbon cloth GDL materials, and GDLs with microporous layers (MPL). The Vulcan carbon catalyst layer bulk resistance is 100 times greater than the bulk resistance of carbon paper GDL (Toray TG-H-120). Carbon cloth (CCWP) has bulk and contact resistances twice those of carbon paper. Compression of the GDL decreases the GDL contact resistance, but has little effect on the bulk resistance. Treatment of the GDL with polytetrafluoroethylene (PTFE) increases the contact resistance, but has little effect on the bulk resistance. A microporous layer (MPL) added to the GDL decreases the contact resistance, but has little effect on the bulk resistance. An equivalent circuit model shows that for channels less than 1 mm wide the contact resistance is the major source of electronic resistance and is about 10% of the total ohmic resistance associated with the membrane electrode assembly.

The Impacts of Image Resolution on Permeability Simulation of Gas Diffusion Layer Using Lattice Boltzmann Method

The effect of image resolution on gas permeability through the x-ray reconstructed carbon paper gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC) was examined in this paper. The 3D models of the GDL at 6 different resolutions were obtained by the x-ray computed tomography imaging technique. Each GDL image was then characterized its gas permeability through the lattice Boltzmann (LB) numerical method. The results suggest that the image resolution has a great impact on gas permeability in both principal and off-principal flow directions. The coarser resolutions can contribute to significant changes in the resulting permeability. However, it can reduce calculation time to a great extent. The results also indicate that the GDL image at the resolution of 2.72 mm provides a good compromise between computation time and accuracy.

The role of the gas diffusion layer on slug formation in gas flow channels of fuel cells

Water drops emerge from large pores of the hydrophobic Gas Diffusion Layers (GDL) into the cathode gas flow channel of Polymer Electrolyte Membrane (PEM) Fuel Cells. The drops grow into slugs that span the cross-section of the flow channels. The slugs detach and are forced out the gas flow channel by the air flow. An acrylic micro-fluidic flow cell with a 1.6 mm gas flow channel and a 100 μm liquid pore through a carbon paper GDL has been used to quantitatively determine slug volumes, velocity of slug motion, and the force required to move slugs as functions of the gas and liquid flow rates. In a channel with 4 acrylic walls, slugs detach immediately upon formation. A porous GDL wall allows gas flow to bypass the slugs, thus allowing slugs to continue to grow after spanning the open area of the channel. The differential pressure to detach and move slugs is equal to the dynamic interfacial force on a slug normalized by the cross-sectional area of the channel. The dynamic interfacial force is equal to the difference between the downstream (advancing) and upstream (receding) contact lines of the water with the channel walls. Slugs will stop moving if the differential pressure drop for gas flow to bypass the slug and flow through the GDL under the rib separating the channels is less than the differential pressure required to move the slug. The results improve our physical insight into the state of water hold up in PEM fuel cells.

Modeling Fluid Flow in the Gas Diffusion Layers in PEMFC Using the Multiple Relaxation‐time Lattice Boltzmann Method

AbstractThe gas diffusion layers (GDLs) are key components in proton exchange membrane fuel cells and understanding fluid flow through them plays a significant role in improving fuel cell performance. In this paper we used a combination of multiple‐relaxation time lattice Boltzmann method and imaging technology to simulate fluid flow through the void space in a carbon paper GDL. The micro‐structures of the GDL were obtained by digitizing 3D images acquired by X‐ray computed micro‐tomography at a resolution of 1.76 μm, and fluid flow through the structures was simulated by applying pressure gradient in both through‐plane and in‐plane directions, respectively. The simulated velocity field at micron scale was then used to estimate the anisotropic permeability of the GDL. To test the method, we simulated fluid flow in a column packed with glass beads and the estimated permeability was found to be in good agreement with experimental measurements. The simulated results for the GDL revealed that the increase of permeability with porosity was well fitted by the model of Tomadakis–Sotirchos [48] without fitting parameters. The permeability calculated using fluids with different viscosities indicated that the multiple‐relaxation time lattice Boltzmann method provides robust solutions, giving a viscosity‐independent permeability. This is a significant improvement over the commonly used single‐time relaxation lattice Boltzmann model which was found to give rise to a unrealistic viscosity‐dependent permeability because of its inaccuracy in solving the fluid–solid boundaries.

Effect of microporous layer on MacMullin number of carbon paper gas diffusion layer

The effect of the microporous layer (MPL) and wet proofing on the MacMullin number has been evaluated for a custom series of Toray TGP-H-060 carbon paper gas diffusion layer (GDL). Complementary characterizations for these GDLs were performed by using scanning electron microscopy (SEM) images, pore size distribution (PSD) and fuel cell performance. The GDLs were customized by the addition of a microporous layer (MPL) and the treatment of, either or both, the substrate and MPL with 10% and 40% hydrophobic agent. SEM images correlated very well with the data shown for PSD. Distinction between the substrate layer and the MPL were clearly shown as two different slopes in the integral distribution and two different peaks in the differential distribution. The MacMullin number increased with increase in wet proofing but decreased with the addition of the MPL. The MacMullin number is a key parameter that contains the missing information for the path length in GDLs, which is generally approximated with the Bruggeman expression. The results provided an overview for the interpretation of the combined effect of the substrate and MPL properties as well as the cell operating conditions.

Transport Phenomena in Carbon Paper Gas Diffusion Layers

The proper transport of water and air through the gas diffusion layer (GDL) is a key characteristic for the efficient operation of polymer electrolyte membrane fuel cells. In this study, the geometry of a carbon paper GDL is generated by implementing an optimized periodic surface model (PS). The fiber distribution in the representative volume element (RVE) is optimized using a genetic algorithm to reduce the complexity of the reconstructed model. The through-plane and in-plane GDL permeability are computed and compared by the experimentally measured data. To investigate the water transport in the fuel cell a segment of the GDL and the air channel in the bipolar plate is reconstructed. The water removal through the three dimensional (3D) reconstructed model is simulated by the volume-of-fluid (VOF) model. It has been found that the water removal in the GDL is due to fingering and channeling mechanism.

Capillary pressures in carbon paper gas diffusion layers having hydrophilic and hydrophobic pores

Capillary pressures in a carbon paper gas diffusion layer (GDL) having hydrophilic and hydrophobic pores of a polymer electrolyte membrane fuel cell (PEMFC) are investigated by both lattice Boltzmann simulations and experimental measurements. The simulated and measured capillary pressures as a function of water saturation for water drainage and imbibition processes in the GDL are presented and compared. It is shown that the pore-scale simulated drainage and imbibition capillary pressure curves are in good agreement with that obtained by experiment, both indicating the coexistence of hydrophilic and hydrophobic properties in the polytetrafluoroethylene (PTFE) treated carbon paper GDLs. The fitted capillary pressure curves, obtained from this paper, can provide more accurate predictions of the capillary pressure in carbon paper GDLs with non-uniform porosity and wettability than the standard Leverett–Udell relationship which was obtained for soil with more uniform porosity and wettability.

Numerical estimation of the effective electrical conductivity in carbon paper diffusion media

The transport of electrons through the gas diffusion layer (GDL) of polymer electrolyte membrane (PEM) fuel cells has a significant impact on the optimal design and operation of PEM fuel cells and is directly affected by the anisotropic nature of the carbon paper material. In this study, a three-dimensional reconstruction of the GDL is used to numerically estimate the directional dependent effective electrical conductivity of the layer for various porosity values. The distribution of the fibers in the through-plane direction results in high electrical resistivity; hence, decreasing the overall effective electrical conductivity in this direction. This finding is in agreement with measured experimental data. Further, using the numerical results of this study, two mathematical expressions were proposed for the calculation of the effective electrical conductivity of the carbon paper GDL. Finally, the tortuosity factor was evaluated as 1.7 and 3.4 in the in- and through-plane directions, respectively.

Pore-scale flow and mass transport in gas diffusion layer of proton exchange membrane fuel cell with interdigitated flow fields

Lattice Boltzmann method (LBM) is employed to investigate pore-scale flow and mass transport in a carbon paper gas diffusion layer (GDL) of interdigitated PEMFC. The carbon paper GDL is reconstructed using the stochastic method, and its macroscopic transport properties are numerically predicted. The predicted anisotropic permeabilities and effective diffusivity of the reconstructed GDL agree well with existing measurements. Then, effects of the porous structures of the carbon paper GDL are explored in terms of fluid flow, species transport and electrochemical reaction. The GDL porous structures greatly affect flow and mass transport, creating distinct specie concentration distribution and local current density distribution. Besides, simulations are performed to explore liquid water behaviors in the reconstructed GDL. The simulation results present a detailed description of the pore-scale liquid water behaviors. Further, simulations are performed to investigate the effects of land width and GDL contact angle on liquid water removal time and residual saturation. Narrower land reduces liquid water removal time and residual saturation. Higher contact angle increases the removal time and reduces the residual saturation.

Measurement of in-plane thermal conductivity of carbon paper diffusion media in the temperature range of −20 °C to +120 °C

Carbon paper is commonly used as the gas diffusion layer (GDL) in polymer electrolyte membrane (PEM) fuel cells as it exhibits high chemical and mechanical durability. This diffusion medium is also anisotropic, which directly affects its transport properties and specifically the thermal conductivity. In this study, the in-plane thermal conductivity of the carbon paper GDL was determined using thermal diffusivity measurements for a temperature range from −20 to +120 °C and four Teflon loadings (0, 5, 20 and 50 wt.%). It is important to understand the effect of temperature on the thermal conductivity since PEM fuel cells are designed to operate under various temperatures depending on the application of use. Further, Teflon is used to change the hydrophobic properties of the carbon paper GDL with 20 wt.% as the most widely used percentage. In this study, the Teflon loadings were chosen to gain a comprehensive understanding of the thermal resistance due to Teflon. In this study, a quasi-steady method was used to measure the thermal properties of the carbon paper; hence, the phase transformation in the presence of PTFE was investigated. The thermal conductivity decreases with an increase in temperature for all samples. The addition of as little as 5 wt.% Teflon resulted in high thermal resistance decreasing the overall thermal conductivity of the sample. Further addition of Teflon did not have major effects on the thermal conductivity. For all treated samples, the thermal conductivity lies in the range of 10.1–14.7 W/mK. Finally, empirical relations for the thermal diffusivity and conductivity with temperature were deduced.

Simulation of liquid water breakthrough in a nanotomography reconstruction of a carbon paper gas-diffusion layer

This study reports the feasibility of newly simulating liquid water intrusion into the porous gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC) using X-ray nano-tomography and two-phase lattice Boltzmann (LB) simulation. A digital 3D model of the GDL is reconstructed using X-ray nanotomography while two-phase porous flow is simulated at two different levels of surface wettability by applying a newly-developed numerical LB model. The results show liquid infiltration in a hydrophobic GDL is comparatively lower (pore saturation of 0.11 to 0.90) than that for a hydrophilic GDL (pore saturation of 0.36 to 0.96) over the liquid intrusion range of 1 kPa . 100 kPa. Visualisation of simulated results in three dimensions reveal dissimilar liquid infiltration characteristics for the two levels of wettability considered, yet also reveal a general breakthrough of liquid water at a pressure of 10 kPa due to specific structural features of the GDL.